The present invention relates to an ink jet printer or like equipment having a liquid supply system installed therein and, more particularly, to a device for continuously detecting the kinematic viscosity of a liquid flowing through the liquid supply system which varies with time.
In an ink jet printer, for example, ink is pumped from an ink supply system to an ink jet head and, by the action of an electrostrictive vibrator, ejected from a nozzle of the head. At a predetermined position ahead of the nozzle, a drop is separated from the jet of ink and charged to a predetermined polarity by a charging electrode. The charged drop is deflected by deflection electrodes to impinge on a recording medium to print out a dot thereon. The prerequisite with the ink jet printer is that drops be separated in a stable manner from the jet to be accurately charged, thereby enhancing quality printout of images. As well known in the art, the separation of drops from an ink jet greatly depends upon the viscosity of ink. Specifically, an ink jet printer of the type described is constructed to recirculate ink by collecting non-printing drops which do not contribute to printout using a gutter. The problem with such a recirculation scheme is that a solvent contained in the ink is evaporated during the flight of drops through the air, with the ink's viscosity gradually increasing in the course of repeated collection. The ink' s viscosity is also susceptible to the ambient temperature. Such variations in the ink's viscosity is reflected by unstable separation of drops from a jet which in turn results in inaccurate charging.
An approach heretofore proposed to maintain ink viscosity constant relies on a flowmeter, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 58-20453/1983. Specifically, a conduit provides fluid communication between an ink reservoir and a pump so that the flow rate of ink circulated through the pipe, which is effected by the resistance of ink due to the ink's viscosity, is measured by the flowmeter. Based on the result of measurement, a solvent is supplied to the ink to control the viscosity to a predetermined one. Such a flowmeter scheme, however, attains only a limited degree of accuracy. In addition, the result of measurement is influenced by fluctuations of pump pressure.
Another approach known in the art uses a float which is dipped in ink so as to measure the specific gravity of the ink, as described in Japanese Unexamined Patent Publication (Kokai) No. 57-12685/1982. This float scheme also fails to accomplish accurate viscosity detection because it detects ink viscosity in terms of specific gravity.